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Former New York City Mayor Michael Bloomberg, Mayor Bill de Blasio, former New York City Mayor Rudy Giuliani, Republican presidential nominee Donald Trump and New Jersey Gov. Chris Christie pay their respects during the 9/11 anniversary service at Ground Zero. (Published Sunday, Sept. 11, 2016)

Hillary Clinton and Donald Trump are attending the Sept. 11 remembrance ceremony in New York to mark the 15th anniversary of the attacks.

The presidential candidates greeted supporters on Sunday as they entered the downtown Manhattan memorial. They're not expected to make public remarks at the event, and both have promised to suspend campaign activities out of respect for the victims, first responders and their families.

Clinton — a former New York senator — has frequently highlighted her efforts, including in a campaign ad released Friday, to aid those affected by the World Trade Center collapse.

Trump — a New York native and real estate mogul — has said he donated construction equipment to the recovery effort and gave $100,000 to the memorial after touring it for the first time earlier this year.

Four hijacked airliners crashed on Sept. 11, 2001 — one into each tower, one into the Pentagon and a fourth in a field in Pennsylvania. Altogether, nearly 3,000 people were killed.

A decade later, U.S. forces killed al-Qaida leader Osama bin Laden during a raid in Pakistan authorized by President Barack Obama. But as al-Qaida's influence waned, the Islamic State group gained strength, exploiting the chaos of a civil war in Syria.

Trump has vowed to defeat the Islamic State group with the advice of military leaders. But he also has said he would suspend Muslim immigration into the United States, a policy he later tried to amend by saying he'd temporarily ban immigration from "areas of the world where there is a proven history of terrorism against the United States, Europe or our allies, until we fully understand how to end these threats."

Clinton, meanwhile, is urging people who like the sound of Donald Trump's boasts about defeating the Islamic State group to consider the difference between "real strength" and "phony strength."

PHOTOS: Nation Marks 15th Anniversary of 9/11 Attacks

"It's phony strength to not know what you're talking about, and to make outrageous statements that will actually make our job harder, no matter how in the moment it sounds," Clinton told CNN in an interview aired Sunday, the 15th anniversary of the attacks on New York and Washington.

"Real strength is leveling with the American people and making it clear we will defeat ISIS," she added. "But that we've got to make sure that here at home, we're not opening doors to people who feel that somehow they want to be part of this global movement because Donald Trump has said it's a war between us and them, and that's pretty attractive to people."